This invention relates to processing of aluminum scrap, e.g. used containers, fabricated, at least in part, from aluminum base alloys containing magnesium. More particularly, this invention relates to a method or process for reducing the magnesium content of such aluminum scrap.
In the packaging or container field, there has been ever-increasing interest and extensive research into methods of reclaiming the aluminum components from scrap materials, such as the aluminum in used beverage containers having at least one or more components thereof fabricated from aluminum alloys. The interest has been precipitated by the importance of conserving resources and caring for environmental problems. However, heretofore recycling such materials has been somewhat hampered by the presence of varying amounts of alloying ingredients in the various alloys contained in the scrap. For example, attempts to recycle a beverage can having a body fabricated from one aluminum base alloy and a top or lid constructed from a different aluminum base alloy often results in an aluminum melt having a composition of neither alloy. Such melt greatly decreases in value because it does not readily lend itself to reuse in the fabrication of new bodies or lids without major dilutions, purifications and realloying, or other modifications.
A notable example of this problem is the varying amount of magnesium which is used respectively in a beverage can body and in the lid or cover for the beverage can. Conventional alloys for can ends, e.g., Aluminum Association (AA alloy) 5182, 5082, 5042, 5352 or 5052, can have up to 4.5 times the amount of magnesium found in the alloys usually used in the can body, e.g., AA3004, AA3104 or AA3003. Thus, when the composite can end and body are subsequently recycled, the overall magnesium content is much lower than that found in the alloys used for can ends but is significantly higher than the amount found in the alloys used for the body of the can. While such scrap metal could be reused in the production of alloys suitable for use in the fabrication of can bodies, it is necessary to carry out costly magnesium reduction procedures and/or dilution of the recovered scrap metal with pure aluminum, that is, aluminum containing only nominal amounts of magnesium.
U.S. Pat. No. 4,269,532 indicates that since the conventional alloys for can ends, e.g. AA5182, AA5082 or AA5052, and for can bodies, e.g. AA3004 or AA3003, differ significantly in composition and in the manufactured can, the end and body are essentially inseparable, and an economical recycle system requires the use of the entire can. U.S. Pat. No. 4,269,632 further notes that the recycling of cans results in a melt composition which differs significantly from the compositions of both the conventional can end and can body alloys. In this patent, it is suggested that both can end and body be fabricated from the same alloy to obviate the recycling problem. With respect to can ends and bodies made from AA5182 and AA3004, it is indicated that some amount of normally pure aluminum must be added even to prepare the special alloy disclosed therein to reduce either the magnesium or manganese levels. The patentees recite the use of conventional shredding and delaquering steps prior to remelt and indicate that the temperature to pyrolyze the organic coatings should not be sufficient to oxidize the metal scrap. It is further indicated by the patentees that, although some amount of magnesium (up to 0.3% may be lost by oxidation in the remelting process, excess amounts of magnesium may be reduced by fluxing the melt with chlorine gas to form an insoluble magnesium chloride but that dilution with pure aluminum is preferred.
It would, of course, be preferred to recover conventional aluminum scrap metal from the melting of containers, such as used beverage containers, regardless of the presence of mixtures of different alloys, which could be recycled back into the manufacture of any alloy which may be subsequently used in the manufacture of either the container body or the lid or cover for the container.